I have a problem with TeXshop (on Sierra). When I click on a
.tex-file it compiles it to PDF but I cannot see (and thus modify) the source window. Any idea why ?
The most likely cause of this behaviour is that you have set TeXShop to be used with an external editor, so TeXShop displays only the preview window, and assumes you are editing the source somewhere else. The source window is therefore unavailable.
To solve this, go to the TeXShop Preferences and in the On Startup part of the panel, make sure the Configure For External Editor is not checked. You will need to restart TeXShop to have the setting take effect.